Ergovaline is a potent vasoconstrictor of isolated bovine uterine and umbilical arteries. Contractions to ergovaline were slow to develop and required 120 min or more to reach their maximum effect. Tissues contracted to ergovaline did not relax over a 3 hr period despite repeated changing of the bath fluid with fresh Krebs solution. This indicates a high affinity of ergovaline for its tissue receptor. Contractions to ergovaline were antagonized by ketanserin and phenoxybenzamine but not by prazosin or phentolamine. This indicates that 5-HT2 receptors but not adrenergic receptors are involved in the vasoconstriction. The results suggest that drugs effective in blocking 5-HT2 receptors may be useful in preventing or reducing the toxic symptoms due to ergovaline in animals consuming Acremonium coenophialum infected tall fescue.